1. Field of the Invention
The present invention relates to frequency converters, but more specifically to circuits having square-law transfer characteristics.
2. Description of the Prior Art
A frequency converter circuit used in a radio receiver is so constructed that a signal having frequency of the sum or the difference of the original signal frequencies is produced from a couple of input signals by using non-linear characteristic of an active element such as a transistor.
The input/output transfer characteristic of a transistor is generally expressed as follows: EQU v.sub.o =a.sub.1 v.sub.i +a.sub.2 v.sub.i.sup.2 +a.sub.3 v.sub.i.sup.3 + . . . +a.sub.n v.sub.i.sup.n ( 1)
in which v.sub.o is output signal, v.sub.i is input signal, v.sub.i.sup.n are harmonics of the input signal v.sub.i, and a.sub.1, a.sub.2, a.sub.3 . . . a.sub.n are constants corresponding to the amplitude of the respective harmonics.
If two input signals having frequencies f.sub.1 and f.sub.2 are applied to this type of circuit, having non-linear characteristic, an output signal whose frequency is expressed as mf.sub.1 .+-.nf.sub.2 (m and n are natural numbers) is produced. This output signal includes so called undesired harmonics, which cause the intermodulation interference.
Especially, it is known that the component of the output signal v.sub.o corresponding to the odd harmonic terms of the equation (1) causes the strong intermodulation interference.
A circuit is therefore proposed by the inventor in which the signal component corresponding to the odd harmonic terms of the equation (1) is decreased to prevent the intermodulation interference.
However, in these types of circuits, the signal component corresponding to the even terms (especially to the fourth term) still remains and also causes intermodulation interference.